As an automatic control method, which controls an amount of operation provided for a controller to follow in target range value under conditions of various state variables such as temperature, pressure, flow, and level of controlled object change in time, PID (proportional, integral, differential) control method is known. PID control is especially used extensively in the field of temperature control (constant temperature control).
FIG. 1 is a flow chart illustrating an embodiment of a conventional temperature control algorithm based on PID control. As shown in FIG. 1, in the temperature control using PID control, a sufficient high-speed response could not be realized, because the amount of operation of the heater is always determined based on a deviation between a target value (target temperature) and a present value (present temperature) processed by operation with respect to each coefficient of proportional, integral, differential elements, and therefore, the amount of operation of every time is large, and requires a long time at operation speed.
Moreover, each control coefficient of proportional, integral, differential elements (Kp, KI, KD) are introduced by mathematical techniques, and are basically not directly connected to a physics characteristic of the temperature-controlled object, and therefore, it is virtually impossible to theoretically seek the most suitable value of each aforementioned control coefficient, and therefore, each aforementioned control coefficient is determined by experiential perception or trial and error. From such a point of view, an accurate and high-speed response thereof could not be realized.
Furthermore, as shown in FIG. 1, the PID control requires an integral element to reach to the target value (target temperature), and therefore, the fact that the pulsation occurs cannot be avoided, which also makes it impossible to realize a sufficiently accurate and high-speed response.
Taking the above-described problems into consideration, various improvements are attempted for PID control. For example, a method for setting a set value is disclosed in Japanese Patent Application Laid-Open No. 2001-92501, in which the amount of control of a controlled object may accurately follow the target value of the controlled object during automatic control by following the target value that changes in time. Such control method has advantages in that it follows a set value precisely according to a characteristic of the system of the controlled object, and a servo-control system may be implemented.
Moreover, a PID control technique is disclosed in Japanese Patent Application Laid-Open No. 2001-163101, in which a PID control technique is characterized by including the steps of: calculating a deviation between a present value which is detected from a controlled object device and a set value which is a target value; operating PID control in response to the deviation calculating step, and then determining whether or not the present amount of operation of the controlled object device is in a predetermined range; outputting a control operation signal to the controlled object device with a PID control operation value multiplied by an output ratio, which is predetermined in response to the amount of operation within the predetermined range, if the present amount of operation is in the predetermined range and a sign of the deviation becomes inverse; and outputting a control operation signal to the controlled object device with a PID control operation value multiplied by an output ratio which is predetermined in response to the amount of operation corresponding to out of the predetermined range, if the present amount of operation is out of the predetermined range.
However, even using above-described PID controls, temperature control with an accurate and high-speed response has not been achieved.